1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus including the fixing device.
2. Description of the Related Art
As a fixing device employed in an image forming apparatus such as a copier, a printer, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices, a thin fixing belt formed of a metal base and a resin rubber surface layer or the like is known. Using such a thin-layered fixing belt with a low thermal capacity can drastically reduce the energy necessary for heating the fixing belt, enabling warm-up time or a first print time (time to first print) to be reduced. Herein, the warm-up time means the time required to raise the temperature of the fixing belt from power-on to a printable state. The first print time is the time required from receipt of a print request to completion of a printing operation and subsequent media discharge.
FIG. 13 shows a conventional fixing device as disclosed in JP-2007-334205-A, which includes an endless belt 100 as a fixing belt; a pipe-shaped conductive member 200 formed of metal disposed inside the endless belt 100; a heat source 300 disposed inside the metal conductive member 200; a pressure roller 400 contacting the metal conductive member 200 via the endless belt 100, thereby forming a nip N between the metal conductive member 200 and the pressure roller 400. The same also discloses that the endless belt 100 rotates accompanied by a rotation of the pressure roller 400 and the metal conductive member 200 guides a movement of the endless belt 100. Further, the heat source 300 inside the metal conductive member 200 heats the endless belt 100 via the metal conductive member 200, and thus, the entire endless belt 100 can be heated. With this structure, the first print time from the heating standby time can be shortened and any shortage of thermal capacity in high-speed printing can be remedied.
JP-2007-233011-A discloses an alternative method to heat the fixing belt directly, without the metal conductive member intermediary, to realize more energy saving and first print time shortening. Thus, as illustrated in FIG. 14, the pipe-shaped metal conductive member is removed from an interior of the endless belt 100. Instead, a planar nip forming member 500 is disposed at a position opposite the pressure roller 400. In this case, because the endless belt 100 can be directly heated by the heat source 300 at a position at which the nip forming member 500 is not disposed, the heating efficiency is drastically improved and the consumed electricity is decreased. With this structure, the first print time from the heating standby time can be further shortened and can result in a cost reduction.
However, if the fixing belt is directly heated as in continuous printing, the temperature of the fixing belt is excessively increased at a portion where the sheet is not passed, that is, a non-sheet passing portion.
JP-2010-66583-A discloses an approach to solve the problem of excessive heating of the fixing belt, in which a shielding member is disposed between the heat source and the fixing belt. The shielding member moves in the sheet width direction so that a heating area of the fixing belt is variably changed and an appropriate heating area is obtained.
However, because the heat source such as a halogen heater has a characteristic in which heating power is reduced at an edge portion thereof, if the heat length is set at the same area as the sheet passing area, the heat distribution is such that the edge portions of the sheet passing area when the printing is started are cooler than the center portion. Accordingly, a heating area of the halogen heater is set to be longer than the sheet passing area of a regular size sheet so that the area with a constant heat power is coincident with the sheet passing area. Thus, fixability at an edge portion even in the first print can be secured. However, if regular size sheets are continuously printed, even though the heat amount in the extended portion of the heater is small, the temperature of the fixing belt is increased excessively and exceeds the permissible range for the fixing belt because heat is not absorbed by the sheet.